Aligned porous materials by directional freezing of solutions in liquid CO2

We report here a method for generating structures with aligned macropores by templating solidified carbon dioxide. The CO2 template phase can be removed by direct sublimation to yield a dry, solvent-free porous material that may be useful in applications such as tissue engineering and aligned cell growth.     

EPR study of molecular complexes in frozen solutions

Conclusions A method was proposed for determining molecular complexes from the EPR spectra of the radical pairs that are formed during the photolysis of frozen solutions. The method has a greater range of application than the ordinary spectrophotometric method.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 625–626, March, 1979.     

Self-assembly of ball-shaped molecular complexes in water

We present a simple and versatile access to spheroidal molecular assemblies with pronounced stability in highly polar solvents. These complexes are composed of doubly and triply charged complementary building blocks based on ammonium or amidinium cations and phosphonate anions. Their high thermodynamic stability is best explained by the formation of a cyclic array of alternating positive and negative charges interconnected by a regular network of hydrogen bonds. Association constants reach 10(6) M(-1) in methanol and often surpass 10(3) M(-1) in water. The broad range of binding energies correlates well with the varying degree of preorganization of both complex partners. As a byproduct of these investigations, new recognition motifs for histidine and arginine esters and the unsubstituted guanidinium ion are proposed. The additional introduction of methyl groups in the 2-, 4-, and 6-positions of the central benzene ring in either cations or anions causes a marked drop in the corresponding K(a) values of 1 order of magnitude; the related rotational barriers were estimated at 0.3-2.1 kcal/mol. Spontaneous formation of defined 2:1 complexes from three components has also been observed.     

Structure of zirconium butoxide complexes in n-butanol solutions

EXAFS and SAXS were used for structure elucidation of zirconium butoxide complexes in n-butanol at concentrations from 0.3 g to 0.015 g ZrO₂ in 1 ml. The basic structural unit of the complex is a tetramer. It has two equal sides with zirconium atoms linked by double oxygen bridges and with zirconium-zirconium distances of 3.5 Å. The other sides in the tetramer are 3.3 Å and 3.9 Å. This difference in bond lengths is explained by the different numbers of double or single ligand bridges between zirconium atoms. The tetramers are apt to undergo oligomerization to form particles with a diameter of 80 Å in solution.     

Hydrogen bonds and molecular complexes in solutions of cellulose nitrate

Intermolecular hydrogen bonds in the systems based on cellulose nitrate and a number of low-molecular solvents were studied by IR spectroscopy. The majority of the systems under study are characterized only by redistribution of intensities of the spectral bands corresponding to the pure polymer accompanied by their minor shift. In this case, acceptors of cellulose nitrate become sterically accessible for the redistribution of hydrogen bonds, and only an insignificant portion of them forms hydrogen bonds with a solvent. New spectral bands in the IR spectra were observed only for solutions containing diethyldiphenyl carbamide, suggesting the formation of a molecular complex. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 3, pp. 484–487, March, 1999.     

Structure:function relationships in molecular spin-crossover complexes

Spin-crossover compounds are becoming increasingly popular for device and sensor applications, and in soft materials, that make use of their switchable colour, paramagnetism and conductivity. The de novo design of new solid spin-crossover compounds with pre-defined switching properties is desirable for application purposes. This challenging problem of crystal engineering requires an understanding of how the temperature and cooperativity of a spin-transition are influenced by the structure of the bulk material. Towards that end, this critical review presents a survey of molecular spin-crossover compounds with good availability of crystallographic data. A picture is emerging that changes in molecular shape between the high- and low-spin states, and the ability of a lattice to accommodate such changes, can play an important role in determining the existence and the cooperativity of a thermal spin-transition in the solid state (198 references).     

Kerr-effect in solutions and molecular polarity of liquid crystalline fluorenes

As part of a broader study of liquid crystal polymers with side groups containing fluorene moieties, more information was required about the properties of fluorene derivatives. With this aim in mind, the solution Kerr-effects for five fluorene-based mesogens have been measured and used in conjunction with other results to determine the polarizability anisotropies and dipole moments.     

Structure and ultrafast dynamics of liquid water: a quantum mechanics/molecular mechanics molecular dynamics simulations study

A quantum mechanics/molecular mechanics molecular dynamics simulation was performed for liquid water to investigate structural and dynamical properties of this peculiar liquid. The most important region containing a central reference molecule and all nearest surrounding molecules (first coordination shell) was treated by Hartree-Fock (HF), post-Hartree-Fock [second-order Moller-Plesset perturbation theory (MP2)], and hybrid density functional B3LYP [Becke's three parameter functional (B3) with the correlation functional of Lee, Yang, and Parr (LYP)] methods. In addition, another HF-level simulation (2HF) included the full second coordination shell. Site to site interactions between oxygen-oxygen, oxygen-hydrogen, and hydrogen-hydrogen atoms of all ab initio methods were compared to experimental data. The absence of a second peak and the appearance of a shoulder instead in the gO-O graph obtained from the 2HF simulation is notable, as this feature has been observed so far only for pressurized or heated water. Dynamical data show that the 2HF procedure compensates some of the deficiency of the HF one-shell simulation, reducing the difference between correlated (MP2) and HF results. B3LYP apparently leads to too rigid structures and thus to an artificial slow down of the dynamics.     

Structure and dynamics of liquid water

A review of recent studies on the structure and dynamics of water (in liquid and amorphous states) by physical methods and computer experiment is presented together with a brief summary of historical background. Basic concepts offered for describing the structure and dynamics of water are considered. The structure of water is currently viewed as having a uniform three-dimensional network formed by hydrogen-bonded molecules. The arrangement of this network is dissimilar from that found in any crystal structures; it is dynamically and structurally inhomogeneous. The network is rather labile; water molecules continuously change their neighbors, so that the average lifetime of hydrogen bonds is about a few picoseconds.     

Change of the luminescence properties of charge-transfer complexes in liquid solutions

The deactivation mechanism of the excited charge-transfer complex in non-polar and polar solvents has been discussed on the basis of measured quantum yields and decay times of fluorescence of charge-transfer complexes of pyromellitic dianhydride with some methylbenzenes. It has been pointed out that great changes of electronic structure in the excited state, caused by strong excited complex—solvent interaction, are responsible for the very low fluorescence quantum yield observed in polar solvents.     

Structure of palladium chloride complexes with organic sulfides in solutions

EXAFS spectroscopy was used to study the influence of various factors on the structure of PdCl₂ complexes with organic sulfides in organic solvents. Absolute interatomic distances in the first coordination sphere of Pd were determined for the complexes [PdCl₂·2(C₆H₁₃)₂S] (I), [PdCl₂·(C₆H₁₃)₂S]₂ (II), [PdCl₂·2(C₆H₅)₂S] (III), and [PdCl₂·(C₄H₉)S(C₄H₇)] (IV) and for their solutions in some organic solvents. Our hypothesis that aromatic solvent molecules are coordinated to palladium atoms through weak π-bonds, which was proposed for complex (I) in benzene, is supported fror benzene and pseudocumene solutions of complexes (I), (II), and (III). It is shown that the characteristic features of the specific solvation of the complexes under study are determined by the electron properties and spatial structures of the molecules as well as by the donating abilities of the solvents. Institute of Inorganic Chemistry, Siberian Branch, Russian Academy of Sciences. Translated fromZhurnal Strukturnoi Khimii, Vol. 36, No. 6, pp. 1030–1037, November–December, 1995. Translated by I. Izvekova     

Structure of liquid solutions near the critical point

Specific features of the structure of the critical state of binary liquid solutions leading to an anomalous behavior of the Rayleigh line due to a dramatic increase in concentration and density fluctuations are considered. It is shown that an experimental treatment must deal with two fluctuation regions near the critical point of solvent vaporization. In the first region, one can achieve a sufficient degree of accuracy by using theories like selfconsistent field theory. In the second region, which is closer to the critical point than the first region, scaling theory of secondorder phase transitions may be applied. It is found that the anomalous behavior of the Rayleigh line associated with kinetic coefficients is determined by the equilibrium thermodynamic properties and by the radius of fluctuation correlation (rinc). A general theory is developed for calculating thermodynamic potentials, especially the chemical potential and its concentration derivative in the fluctuation region. The results of these calculations are compared with the experimental data briefly described in the paper. Translated fromZhumal Strukturnoi Khimii, Vol. 39, No. 4, pp. 655–668, July–August, 1998.     

Infrared spectra of CO2-H2 complexes

Infrared spectra of weakly bound CO(2)-H(2) complexes have been studied in the region of the CO(2) v(3) asymmetric stretch, using a tunable diode laser probe and a pulsed supersonic jet expansion. For CO(2)-paraH(2), results were obtained for three isotopic species, (12)C(16)O(2), (13)C(16)O(2), and (12)C(18)O(2). These spectra were analyzed using an asymmetric rotor Hamiltonian, with results that resembled those obtained previously for OCS- and N(2)O-paraH(2), except that half the rotational levels were missing due to the symmetry of CO(2) and the spin statistics of the (16)O or (18)O nuclei. However, for CO(2)-orthoH(2), more complicated spectra were observed which could not be assigned, in contrast with OCS- and N(2)O-H(2) where the paraH(2) and orthoH(2) spectra were similar, though distinct. The CO(2)-paraH(2) complex has a T-shaped structure with and intermolecular distance of about 3.5 Angstroms, and the CO(2) v(3) vibration exhibits a small redshift (-0.20 cm(-1)) in the complex.     

Structure formation and gelation phenomena in solutions of ternary interpolyelectrolyte complexes

The representative of the new family of mechanically reversible gels is described. The gel is formed by mixing of an aqueous solution of non-stoichiometric interpolyelectrolyte complex of poly (sodium methacrylate) and poly(N-ethyl-4-vinylpyridinium bromide) containing a certain amount of covalent links between oppositely charged polyions, with aqueous solution of poly(potassium vinylsulfate). The gelation mechanism arises to the partial replacement of the electrostatic contacts between the polycation and poly(methacrylate) anion in the original polycomplex with those between the polycation and poly(vinylsulfate) polyanion. The network of the gel is most probably formed by the nodes consisting of covalent links and interpolyelectrolyte double-strand electrostatic clusters, united by poly(sodium methacrylate) tie-chains.     

Chemical transformations of CO2 in trifluoroacetic acid solutions

It is established that a conversion reaction of carbon dioxide takes place at room temperature and atmospheric pressure in aqueous solutions of trifluoroacetic acid (TFA), which leads to the formation of oxalic acid and heavier polymerized products.     

Electrocatalytic reduction of CO2 to CO by polypyridyl ruthenium complexes

Electrocatalytic reduction of CO(2) by [Ru(tpy)(bpy)(solvent)](2+) (tpy = 2,2':6',2'-terpyridine, bpy = 2,2'-bipyridine) and its structural analogs is initiated by sequential 1e(-) reductions at the tpy and bpy ligands followed by rate limiting CO(2) addition to give a metallocarboxylate intermediate. It undergoes further reduction and loss of CO.